Two beginner questions about align correctly
Assuming there are no predetermined measurements

Alignment I


In an empty scene create a cube and scale it down on the X and Y axis only to about half the original, by press Shift + Z.





Then in edit mode, switch to Right view by pressing Num3, select both the top and bottom edges and scale down on the Z axis by about 0.5.





Add three vertical loop cuts and adjust the other two closer to the middle one by select one of edge, then tapping G twice to enter Edge Slide Mode and sliding it closer to the middle loop.







Delete the faces around the middle loop. Then select the edges around the open part and create a face by pressing F for both the segments.


 



Alignment II


While still in edit mode, enter Face Select Mode and select the top face of the larger segment and press Shift + S and select cursor to selected to align the cursor to the point.





Then create a cube and scale it down to the desired size. Select only the newly created cube completely and rotate it around the X axis, while using the "face dots" to check your alignment. press P and select "By loose Parts".





Select that cube, and in edit mode, select Normal Orientation and select the top face and pull it using the manipulator along its normal Z axis. Repeat the same step for the bottom face till it is at the distance you want it to be from the other surface.




NOTE: If there are any predetermined measurements feel free to alter the scaling transformations as you see fit.

Hope this helped.